Paleozoic Partner Highlight

Falls of the Ohio State Park contains a spectacular exposure of Devonian-age limestone that displays millions of fossils from hundreds of species. In the photo, visitors explore the lower fossil beds in September. Photo courtesy Falls of the Ohio State Park/IDNR.

Envisioning a warm tropical sea in Indiana requires some imagination, particularly if you visit in January and experience how "un-tropical" it is! However, geological evidence shows that the North American continent was south of the equator during the Middle Devonian Period, 390 million years ago, and beneath a shallow sea. Fossil evidence shows that this sea was teeming with life. The 200 acre fossil beds within Falls of the Ohio State Park in Clarksville, Indiana, is the best exposure of its type in the United States. The limestone exposures within the park are chock full of fossils and many are in situ—in the position in which they grew 390 million years ago.

Not only has the position of North America changed, but so has the nature of the landscape. Or should we say, seascape! Middle Devonian rock exposures from New York to Iowa and south to Tennessee reveal a diverse population of marine invertebrate fossils. In places such as those within and surrounding Falls of the Ohio in southern Indiana and north-central Kentucky, as many as 600 species have been documented! These limestone layers stretch north and east where the Jeffersonville Limestone is called the Columbus Limestone (Ohio) and Onondaga Limestone (New York), and west as the Grand Tower Limestone (Illinois). In addition to containing the best exposure of its type in the country, what makes the Falls of the Ohio unique is that it is easily accessible, located in the middle of a metropolitan area with more than a million people.

How does Devonian sea life compare to today? If you could snorkel in that ancient sea, you would observe corals, sponges, shells, and many other creatures with similarities to those living now. But a closer look would reveal some startling differences—corals shaped like cow horns, large mound-shaped sponges seemingly made out of rock, brachiopod shells with a symmetry that is different from clams, there would be fewer fish, and large marine "pill bugs" (trilobites) instead of crabs. Waving slowly in the currents would be peculiar "flowers"—crinoids—that seem to be made of colored stone. Many of these animals are featured on the 2013 National Fossil Day artwork.

The Devonian is sometimes called "The Age of Fish" because most of the diversity of modern and extinct fish originated in that time period. However, complete fossil fish are extraordinarily rare. Teeth, scales, spines, and assorted bones are generally found in bone beds rather than distributed evenly throughout the rock layers. Paleozoic invertebrate fossils, such as those found at the Falls of the Ohio, are much more common.

Hundreds of marine invertebrate fossils are known from Falls of the Ohio State Park and the surrounding area. Here are just three. From left to right: "Pipe organ" coral Acinophyllum (lower fossil beds); a well-preserved "Staghorn" coral Thamnopora (lower fossil beds); and a shell bed with the large snail Turbinopsis and numerous brachiopods and small horn corals (upper fossil beds). Photos courtesy Falls of the Ohio State Park/IDNR.

At the Falls of the Ohio, the fossil beds are accessible to most visitors year round, except for a few weeks in the winter and spring when the Ohio River is high. There are two distinguishable areas at the Falls: the coral beds (lower fossil beds) and the upper fossil beds. The latter are exposed most of the year and contain the greatest diversity of phyla including Arthropoda (trilobites), Brachiopoda, Bryozoa, Cnidaria (corals), Cnidaria (corals), Echinodermata (crinoids and blastoids), and Mollusca. The upper fossil beds are layers containing either millions of individual brachiopod shells belonging to a single species, or strata with an abundant mixture of brachiopods and bryozoans.

The lower fossil beds in the floor of the Ohio River are usually exposed from mid-summer through autumn. Because the Ohio River drains a good part of the eastern United States west of the Appalachian Mountains, a widespread rain can cause the river levels to rise and cover the coral beds. Conversely, when weather is drier than normal, the beds are exposed earlier.

Coral beds at the Falls of the Ohio, downtown Louisville in the background. Note the large Acinophyllum coral colony in the lower left. Photo courtesy Falls of the Ohio State Park/IDNR.

The coral beds are dominated by the phyla Cnidaria and Porifera. The fossil beds contain numerous examples in situ. Walking on this layer is like "dry snorkeling." You can see fossils from a few millimeters (if you get on your hands and knees and use a scrub brush) to more than 10 meters (30 feet) across! There are horn corals as long as your arm, including the largest horn coral in the Western Hemisphere, measuring more than 1 meter (more than 3 feet) long. Honeycomb coral (Favosites) colonies are as large as 3 to 15 meters (10 to 50 feet), though most are under a meter (about 3 feet). Colonial rugose corals (locally called pipe organ coral) are often a meter across. Petoskey stones, a catch-all term for Prismatophyllum can run 1 to 3 meters (3 to 10 feet) across.

When the river is at its lowest level, the rock layers reveal thousands of three-dimensional fossils, replaced by silica and more resistant to the river's erosive force. Blackened by a coating of manganese, these fossils contrast beautifully with the surrounding buff-colored limestone. Superimposed are numerous potholes formed by the scouring action of sand and gravel, giving this part of the fossil beds the moniker 'craters of the moon.'

The Interpretive Center at the Falls of the Ohio State Park has served visitors since 1994 with exhibits that tell the story including a "living" Devonian sea, examples of fossils, and many aspects of the geological history. The park website has many educational resources, including a virtual tour of the fossil beds, interactive fossil brochures, and much more.

Article by Alan Goldstein (Falls of the Ohio State Park, Interpretive Naturalist)